Mode choice for zero-distance trips

[sebhoerl]

Sometimes, we can have zero-distance trips in the population. This happens especially because of the way work and education locations are treated right now in the pipeline (always at the same spot for one particular agent). It would be good to make sure that those trips are always "walk", to avoid confusion (and not skew results, for instance on the mode shares), or filter them out in the analysis. A first step would be to check if we can enforce zero-distance trips to be "walk". Does this break the vehicle-based constraint logic? It should not, because if a vehicle is brought to the first activity, it is still at the same location when departing from the second activity at the same spot. Basically, this is a walking subtour, but should be checked!

• Enforce walk for zero-distance trips (or maybe even introduce a mode called none ?)
• Check that this has no impact on the vehicle tour constraints (breaking mass conservation)